1. Field of the Invention
The present invention relates to an oxygen-absorbing resin composition which contains an oxygen absorber and a thermoplastic resin. This invention also relates to a packing material composed of this resin composition. Furthermore, this invention relates to a package and a packing method for preserving a substance in a dried state with a low water content (mainly, dried food) by utilizing the above-mentioned packing material.
2. Description of the Prior Art
An oxygen absorber which utilizes an oxidation reaction of a reduced metal has been widely known. Other than the reduced metal, this oxygen absorber comprises a metal halide as an accelerator for promoting the oxidation reaction of the reduced metal.
In order to mainly preserve the freshness of food, this type of oxygen absorber is placed together with a preserved substance such as food in a container or a packing material, or the oxygen absorber is contained within a container or a sheet material, which is then used to package a substance to be preserved such as food.
The Japanese Patent (Kokoku) Publication No. SHO 56-33980 discloses an oxygen absorber of metal powder coated with a metal halide. Since a deoxidation reaction of a reduced metal requires the existence of water, this type of oxygen absorber is used to preserve a food containing a large amount of water, which is capable of utilizing water transpired from the preserved substance.
On the other hand, in order to preserve a substance having a low water content (i.e., having a low water activity (a.sub.w)), such as dried food, a water-supplying component is included in an oxygen absorber in order to supplement water necessary for the oxygen-absorbing reaction. This type of oxygen absorber is known as a self-reacting type which is capable of absorbing oxygen without depending on water from the preserved substance.
The recent practice has been to package a preserved substance by using a sheet-shaped packing material which is made by kneading an oxygen absorber with a resin and forming the kneaded mixture into a sheet or film. However, if the self-reacting type of oxygen absorber which holds water is used to preserve a substance having a low water content, the transfer of water from the water-holding component to the preserved substance cannot be prevented. This results in problems in that the water spoils the taste of the preserved substance (due to humidity), changes the properties of the preserved substance (that is, solidification of powder), causes a chemical change (or hydrolysis), or causes propagation of bacteria. In some case, another problem is caused in that the oxygen absorber loses the water necessary for deoxidation and then becomes incapable of absorbing oxygen.
Furthermore, as the self-reacting type of oxygen absorber which holds water is heated when it is kneaded with a resin in order to be formed into a sheet, there is a problem in that not only water in the water-holding component evaporates, but also the evaporated water generates bubbles within the resin sheet, the surface of the resin sheet is made uneven and good surface properties of the sheet cannot be obtained.
On the other hand, when an oxygen absorber holding no water (water-dependent type) is kneaded with a resin to obtain a sheet material, it is necessary to perform the treatment for making the oxygen absorber hold water. However, if the resin sheet is made to hold water, this oxygen-absorbing resin sheet takes over the above-described defects of the self-reacting oxygen absorber.
Accordingly, the conventional oxygen-absorbing sheet is limited for the practical use to the type which itself contains no water and which is capable of absorbing oxygen by acquiring water from a food having a high water content which contains moisture necessary for the oxygen-absorbing reaction. This oxygen-absorbing sheet is usually applied to a food containing a large amount of water.
Because of the reasons described above, those skilled in the art have had difficulties in realizing an oxygen absorbing sheet capable or causing the deoxidation even in a low-water environment. In other words, it has been difficult to package a dried food, that is, a substance having a low water content, in good condition in a low oxygen environment by using the conventional oxygen absorber.
Through diligent studies to solve the above-described problems, the inventors of this invention have found that iodide and bromide, which have been completely disregarded as being inferior as an accelerator for the oxygen-absorbing reaction to chloride, such as potassium chloride or sodium chloride, can surprisingly and sufficiently function as the accelerator for the oxygen-absorbing reaction to preserve a packaged substance (or preserved substance) having a low water content by requiring the existence of almost no water-holding component.
The aforementioned Japanese Patent (Kokoku) Publication No. SHO 56-33980 lists iodide and bromide as one of the accelerators for the oxygen-absorbing reaction, but does not disclose the oxygen absorber which has an oxidation accelerator containing a metallic salt of iodine or bromine as its principal component and which is appropriate for the preservation of a substance having a low water activity.
Although the Japanese Patent (Kokoku) Publication No. HEI 2 22701 discloses an oxygen absorber which is composed of a halide of basic anion exchange resin and an iron powder and which does not require the use of water soluble substances, it does not aim at providing a halide as an oxygen absorber capable of functioning even in a low humidity atmosphere and in a dry condition. It does not actually consider that a metallic salt or iodine or bromine can fully function as an accelerator in a dry condition.
Accordingly, the object of the present invention is to provide an oxygen-absorbing resin composition capable of sufficiently exhibiting the oxygen-absorbing function even in the low humidity atmosphere. Another object of this invention is to provide an oxygen-absorbing resin composition having good thermoformability.
A further object of this invention is to provide an oxygen-absorbing member and packing material in the form of a sheet or film, and a packing container, which are capable of exhibiting a sufficient oxygen-absorbing function in the low humidity atmosphere.
Another object of this invention is to provide a package to preserve a substance having a low water content, which requires the low-humidity preservation condition, in the atmosphere of low oxygen concentration. A further object of this invention is to provide a method for a preserving a substance having a low water content, which method makes it possible to maintain the substance having a low water content in the atmosphere of low oxygen concentration by using the above-mentioned oxygen-absorbing resin composition.